1. Field of the Invention
The present invention generally relates to an exposure apparatus used in a printer, a facsimile, a copying machine, and the like, and also to an optical write head employed in such an exposure apparatus. More specifically, the present invention is directed to such optical write head/exposure apparatus capable of writing data in a high speed without fluctuations in light emitting timing.
2. Description of the Related Art
Conventionally, in image forming apparatuses such as printers, latent images are formed on the surfaces of photosensitive members, the latent images are developed by using developing agent, and images developed by the developing agents are transferred to paper, and thereafter the transferred images are fixed on the paper by using fixing devices.
To form the latent images on the surfaces of the photosensitive members in such images forming apparatuses, exposure apparatuses are provided. These exposure apparatuses utilize LED (Light Emitting Diode) arrays and laser beams. In an exposure apparatus with employment of such an LED array, a plurality of LED elements, the total number of which is defined so as to achieve necessary resolution, are arranged in a predetermined range, so that an LED array (namely, optical write head) is formed.
FIG. 7 schematically indicates an arrangement of one conventional optical write head. As shown in this drawing, this conventional optical write head is constituted by a shift register 11, a register 12, and a large number of LEDs 13. To the shift register 11, 16-bit parallel data DATA00 to DATA15, a shift clock CLK, and a start signal DSTAR are inputted. Both an output from this shift register 11 and a latch signal LST are entered into the register 12. A large number of LEDs 13 are arranged in one line, by which a 1-line exposuring operation can be performed. Furthermore, the conventional optical write head is arranged by a switching element 14 and an AND gate circuit 15. The switching element 14 turns ON/OFF the LEDs 13. The AND gate circuit 15 AND-gates the output from the register 12 with a drive signal DSTB, and supplies the AND-gated signal as a control signal to the switching element 14.
When the 1-line exposing operation is carried out, the start signal DSTR is first inputted to this optical write head. Then, the image data for 1 line are entered to the optical write head as the parallel data DATA00 to DATA15 in unit of 16 bits in synchronism with the shift clock CLK. Thereafter, when the supply of the image data for 1 line is accomplished, the latch signal LST is entered, and also the drive signal DSTB is entered.
When the start signal DSTR is entered to the shift register 11, this shift register 11 starts to acquire the parallel data DATA00 to DATA15. Every time the shift clock CLK is inputted thereto, this shift register 11 shifts the parallel data stored therein, and also acquires new parallel data DATA00 to DATA15. When the latch signal LST is inputted into the register 12, this register 12 latches the data for 1 line which are outputted from the shift register 11, and then outputs the latched data. Upon receipt of the drive signal DSTB, the AND gate circuit 15 into which the data outputted from the shift register 11 is inputted supplies such a signal having the same level as that of the signal derived from the shift register 11 to the switching element 14. As a result, only LEDs 13 emit the light, and these LEDs 13 correspond to the bits of the shift register 11 from which high level signals are outputted. Then, while this condition is maintained for a preselected time period, the 1-line exposing operation responding to the inputted image data is carried out. Thereafter, the image data as to the next 1 line are supplied in a similar process operation in order to execute the next 1-line exposing operation.
As apparent from the foregoing descriptions, when an optical write head is manufactured in such a manner that a total number of LEDs which constitute 1 line is increased and the circuit arrangement shown in FIG. 7 is employed, such a time duration directly proportional to this total number of LEDs is necessarily required so as to set image data for 1 line.
Accordingly, other types of optical write heads have been proposed. That is, in order to shorten the time duration required to set the image data for 1 line, the optical write head constructed of shift registers, LEDs and the like is subdivided into a plurality of groups. The image data can be transferred in a parallel manner to the shift registers belonging to each of the subdivided groups. For instance, Japanese Unexamined Patent Application No. 58-48562 opened in 1983 discloses such an optical write head that the LEDs are subdivided into a plurality of LED groups, and the control circuits used to drive LEDs belonging to a subdivided LED group are employed with respect to each of these LED groups. However, in this type of optical write head having the above-described subdivided LED groups/control circuits, although the image data can be set in high speed, the light emitting timing is fluctuated. As a result, there is a problem that the print effects could not be obtained under better conditions.